OXCGN wrote an article a while back about the death of arena shooters, and it talks about this somewhat.

Halo is one of the last big remnants of the arena shooter, and adding iron sights would kill that. Arena shooters are meant to have players running and gunning, not stopping to aim every time they get into a fight.

Obviously Halo has sniper rifles and the DMR/BR, but because they have scopes, it doesn't really detract from the action. If everything had...